Vasopressin V₁-Receptor Assay in Rat Small Mesenteric Arteries

Abstract

The bioassay of vasoactive peptides has often been confined to in vivo preparations because of the lack of receptors on the traditional, robust, functional assay of an isolated large artery preparation as in the rabbit or rat aorta. The standard preparation for the bioassay of arginine vasopressin (AVP) and structural analogues, especially for the development of antagonists, has been the acute rise in blood pressure to two bolus intravenous doses of arginine vasopressin (1x and 2x) in urethane anaesthetized rats pretreated with phenoxybenzamine to antagonise α_l- and α₂-adrenoceptors (1). After pretreating the rats with a potential antagonist, the 1x and 2x doses of arginine vasopressin are repeated. The “effective dose” of the antagonist is then defined as the dose (nanomoles per kilogram) that reduces the rise in pressure from 2x dose of agonist to the response observed for 1x in the absence of the antagonist. By taking the volume of distribution as 67ml/kg body weight (assuming one compartment kinetics) for the antagonist, one can readily calculate a “pA₂” value (-logM) for this concentration. This type of analysis in vivo assumes that (i) the antagonist drug has achieved equilibrium in the receptor biophase (ii) that the antagonist has shifted the agonist dose — pressor response curve in parallel without change in maximum and that the reduction in response from 2x to 1x dose of AVP equates with a two fold shift to the right in agonist (AVP) sensitivity. Moreover, the continued rightward shift in agonist curves with increasing dose levels of antagonist is not tested.